Adult medulloblastomas are uncommon primitive neuroectodermal tumors. [1] Preoperative radiology is usually suggestive. However, imaging at times may be confusing and mimics a low-grade lesion like Lhermitte-Duclos disease (LDD). [1],[2] We report two patients of medulloblastoma radiology mimicking LDD.

Patient 1

A 28-year-old male presented with features of raised intracranial pressure of 2 months duration. Non-contrast computed tomography (NCCT) scan revealed obstructive hydrocephalus with a posterior fossa mass with no calcification or hemorrhage. Magnetic resonance imaging (MRI) revealed a diffuse increase in the bulk of cerebellum, altered cerebellar architecture, and the characteristic "tiger-striped" pattern on T2-weighted image (T2WI). A contrast study showed no enhancement of areas of abnormal signal intensity except the linear enhancement along the cerebellar folia and sulci, secondary to congested vessels. Diffusion-weighted imaging (DWI) did not show any restriction. The features were suggestive of LDD. However, MR-spectroscopy (MRS) revealed raised choline levels and Cho:Cr and Cho:NAA ratios of 1.44 and >30 respectively, suggesting a mitotic pathology [Figure 1]. He underwent ventriculo-peritoneal shunt followed by midline suboccipital craniectomy and decompression of left cerebellar hemisphere. The cerebellum was swollen, tense, with broadened and pale gyri. Histopathology and immunohistopathology confirmed World Health Organization (WHO) grade 4 medulloblastoma. Cranio-spinal imaging did not reveal any evidence of cerebrospinal fluid (CSF) seeding. The patient received cranio-spinal irradiation of 36Gy with 18Gy boost to the posterior fossa. The patient did not give consent for chemotherapy and expired 8 months post-operatively.

Patient 2

A 19-year-old male presented with repeated episodes of vomiting for 2 months. NCCT showed diffuse cerebellar enlargement without any calcification or hemorrhage. MRI revealed diffuse and massive cerebellar enlargement with both upward and tonsillar herniation. The typical striated pattern of LDD was appreciated in both cerebellar hemispheres, with a small enhancing nodule located posteriorly near the torcula [Figure 2]. MRS revealed a mitotic pathology. He underwent a midline sub-occipital craniectomy to decompress the posterior fossa, with excision of the enhancing nodule and a small portion of the adjacent vermis. Vomiting subsided following surgery. Histopatholgy and immuno-staining revealed WHO grade 4 medulloblastoma. Cranio-spinal imaging did not reveal any CSF or meningeal seeding. He received cranio-spinal irradiation, and is doing well at 6 months follow-up.

Figure 2: Cranial MR imaging showing typical striated appearance of both the cerebellar hemispheres T1 and T2. There is no contrast enhancement expect for a small midline nodule and linear hyperintensities along the sulcal space. Midsagittal section reveals both upward and tonsillar herniation

Adult medulloblastomas are uncommon, accounting for 1% of adult central nervous system neoplasms. [1] Unlike children, adult medulloblastomas are more likely to be hemispheric than midline, with cystic or necrotic components, poorly defined margins and relatively less contrast enhancement. [1]

LDD or dysplastic gangliocytoma is a rare disorder, characterized by a cerebellar mass of abnormal ganglion cells producing circumscribed regions of enlarged cerebellar folia and is considered a phakomatosis with autosomal dominant inheritance, caused by a mutation of the PTEN suppressor oncogene. [3] Imaging demonstrates unilateral cerebellar enlargement with a gyriform pattern within the lesion corresponding with the enlarged folia. On MRI, the lesion is hypointense on T1WI and hyperintense on T2WI with the preserved folial pattern producing a striated "tiger-striped cerebellum" pattern that is characteristic of this tumor. [1],[2],[3],[4],[5] The lesion classically does not enhance, but there may be occasional peripheral enhancement due to proliferation of veins near the edge of the tumor in the molecular layer and overlying leptomeninges. Spectroscopy shows decreased NAA/Cr ratios and no increase in the Cho/Cr ratio. [3]

Most cerebellar masses destroy the folial pattern and reveal enhancement, but the non-enhancing cerebellar hemisphere mass with the characteristic striated magnetic resonance appearance (thickened cerebellar folia) in a patient with dysplastic gangliocytoma is distinctive and suggests the diagnosis of LDD. The findings were supposed to be characteristic enough, either obviating the need for an obligatory biopsy for asymptomatic patients or permiting more definitive planning for surgical decompression in restricted lesions. [3] Management of LDD is observation till mass effect by the lesion necessitates decompression. [3] Medulloblastoma, on the other hand, is a malignant neoplasm that essentially requires gross total excision followed by radiotherapy and chemotherapy in order to achieve a good outcome. Thus, pre-operative diagnosis plays an important role.

MRI in both our patients showed the characteristic laminated cerebellar appearance without enhancement. The only odd point was MRS showing a mitotic pathology. Another finding of interest was that both our patients showed the classic medulloblastoma on histopathology and not the commonly found desmoplastic variant seen in this age group. These patient highlight the fact that the diagnosis of posterior fossa masses cannot be purely based on imaging studies as adult medulloblastomas can have myriad presentations, even mimicking hamartomatous lesions like LDD. Thus, a histopathological diagnosis is a must, though spectroscopy may help. Any deviation from MRS findings characteristic of LDD should prompt a biopsy.